Increasing applications of various thermoplastic resins enhances the request to decrease specific density (specific gravity) of resin materials in view of cost reduction, weight reduction, response rate improvement, and other properties. Under the situation, there are provided methods to decrease the specific density of resin by introducing air or inert gas into the resin. In particular, a technology of compounding a hollow filler in the resin has become a common practice. Also to liquid crystalline resins, the technology of compounding a hollow filler therewith has been provided (JP-A 2001-172479). The technology has an additional effect of decreasing the permittivity, as well as decreasing the specific density, thereby further widening the application field of the liquid crystalline resins (JP-A 2004-27021).
However, if the technology proposed in the related art is applied to compound a hollow filler with a resin using common melt processing extrusion process, large percentage of hollow filler may be broken owing to the shearing force of melting-kneading, thus the technology is substantially difficult to attain a composition of low specific density.
Although a hollow filler has a certain level of pressure resistance, excess pressure readily breaks the hollow filler to fail in attaining a desired effect (decreased specific density) which is a target of compounding the hollow filler with the resin. Normally break of hollow filler to some extent is unavoidable owing to the stress history during the manufacturing process. Since the residual percentages of hollow filler in the resin composition, however, significantly affect the decrease in the specific density of the composition, it is desirable to control the manufacturing conditions to maintain a high residual percentage of hollow filler.
Dispersion of hollow filler in the resin composition gives an apparent structure containing fine bubbles therein, which deteriorates extremely the resin physical properties such as flexural strength and flexural modulus, though the specific gravity becomes small. As a result, the resin composition with dispersed hollow filler often loses the practical applicability in terms of stiffness and other properties unless a fibrous filler such as glass fiber is further added. The above-cited JP-A2001-172479 and JP-A 2004-27021 also recommend the parallel use of a fibrous filler. If, however, a fibrous filler is added, the break of hollow filler during the manufacturing process may further increase owing to the increase in the viscosity and the increase in the occasions of collision of filler.
The fibrous filler such as glass fiber is preferably dispersed in the resin composition in a length range from about 200 μm to about 700 μm, more preferably from about 300 μm to about 600 μm. The preferable range is derived based on a situation that the articles of moldings requiring decreased specific gravity, using the composition, are generally in small size and having thin thickness portions so that excessively large fiber length may often induce problems such as insufficient filling, or excessively small fiber length fails to attain sufficient stiffness. For dispersing glass fiber into the resin composition at above-described fiber lengths, it is necessary to break the glass fiber applying a certain stress thereto during the manufacturing process. The stress, however, causes the decrease in the residual percentage of the hollow filler.
Consequently, general melting-kneading process adopted in the related art is difficult to maintain a good balance between the residual percentage of hollow filler and the length of glass fiber.
To solve the problem, JP-A 2001-310323 provides a method to increase the residual percentage of hollow filler by feeding a thermoplastic resin from a main feed port located at upstream side in the extruding direction, while feeding a hollow filler from a side feed port located at downstream side in the extruding direction, (if an inorganic fiber is simultaneously used, it may be fed from either the main feed port or the side feed port). JP-A 2001-310323, however, describes only the increase in the residual percentage compared with the percentage attained from simultaneous feed of the thermoplastic resin and the hollow filler from the main feed port, and the method does not consider the influence on the physical properties. As a result, the effect of the method is not sufficient.
An ideal method to maintain a good balance between the residual percentage of hollow filler and the length of glass fiber is to feed a liquid crystalline resin from the main feed port to sufficiently plasticize it, and then to feed a glass fiber from the side feed port to fully knead them together, and finally to feed a hollow filler from a separate side feed port to disperse into the resin composition. Actual melting-kneading extruders, however, have problems of limited extruder length, and of locating plurality of side feed ports, thus the method cannot be applied to common apparatuses.